This invention relates to a magnetic recording medium.
Today the magnetic recording medium in varied forms commonly consists of a backing material coated with a mixture of magnetic powder and binder.
For that purpose a variety of binders have hitherto been proposed, but none have yet been described which permit the resulting medium sufficiently to meet both of two major requirements of such media, namely acceptable electromagnetic conversion characteristic and physical properties such as stability on repeated runs and bonding strength of the coating.
The binders thus far employed have in large measure been urethanes, vinyl chloride-vinyl acetate copolymers, cellulose plastics, and epoxy resins, principally combinations of two such components. The deficiency common to the binary binders has been their inability to provide a medium which meets the two above described requirements in a satisfactory manner.
The magnetic coating on a recording medium is abraded as the medium is repeatedly drawn past the magnetic head, with the coating in sliding contact with the head. Because of its effectiveness in increasing the resistance of the coating to this wearing action, vinyl chloride-vinylidene chloride copolymers, are presently favored. However, coating materials utilizing such copolymers manifest the disadvantages of gelation and embrittlement of the coated film with time. To improve the coated film quality, mixing the copolymer with an epoxy resin and a isocyanate terminated polyurethane resin was proposed (Japanese patent application Public Disclosure No. 160013/1975). The proposal makes use of the cross linking by the reaction of the --OH group of the epoxy resin with the isocyanate group (--NCO). The use of this ternary binder has somewhat improved abrasion resistance, but the products are not completely satisfactory. (The numerical values of wear resistance, i.e., of repeated running stability, tabulated in the published specification are much lower than those of the present invention as will be seen hereinafter.)